Nanotechnology involves the ability to engineer novel structures at the atomic and molecular level. One area of nanotechnology is to develop chemical building blocks from which hierarchical molecules of predicted properties can be assembled. An approach to making chemical building blocks or nanostructures begins at the atomic and molecular level by designing and synthesizing starting materials with highly tailored properties. Precise control at the atomic level is the foundation for development of rationally tailored synthesis-structure-property relationships which can provide materials of unique structure and predictable properties. This approach to nanotechnology is inspired by nature. For example, biological organization is based on a hierarchy of structural levels: atoms formed into biological molecules which are arranged into organelles, cells, and ultimately, into organisms. These building block capabilities are unparalleled by conventional materials and methods such as polymerizations which produce statistical mixtures or confinement of reactants to enhance certain reaction pathways. For example, from twenty common amino acids found in natural proteins, more than 105 stable and unique proteins are made.
One field that will benefit from nanotechnology is filtration using membranes. Conventional membranes used in a variety of separation processes can be made selectively permeable to various molecular species. The permeation properties of conventional membranes generally depend on the pathways of transport of species through the membrane structure. For example, while the diffusion pathway in conventional selectively permeable materials can be made tortuous in order to control permeation, porosity is not well defined or controlled by conventional methods. The ability to fabricate regular or unique pore structures of membranes is a long-standing goal of separation technology.
Resistance to flow of species through a membrane may also be governed by the flow path length. Resistance can be greatly reduced by using a very thin film as a membrane, at the cost of reduced mechanical strength of the membrane material. Conventional membranes may have a barrier thickness of at least one to two hundred nanometers, and often up to millimeter thickness. In general, a thin film of membrane barrier material can be deposited on a porous substrate of greater thickness to restore material strength.
Membrane separation processes are used to separate components from a fluid in which atomic or molecular components having sizes smaller than a certain “cut-off” size can be separated from components of larger size. Normally, species smaller than the cut-off size are passed by the membrane. The cut-off size may be an approximate empirical value which reflects the phenomenon that the rate of transport of components smaller than the cut-off size is merely faster than the rate of transport of larger components. In conventional pressure-driven membrane separation processes, the primary factors affecting separation of components are size, charge, and diffusivity of the components in the membrane structure. In dialysis, the driving force for separation is a concentration gradient, while in electrodialysis electromotive force is applied to ion selective membranes.
In all these methods what is required is a selectively permeable membrane barrier to components of the fluid to be separated.